Currently Mdaemon processes messages through MDSPAMD one at a time, even though there are settings in MD that suggest you can set how many are processed at once. On a medium to high volume server, processing one at a time causes the messages to pile up very quickly. Even with finely tuned SA rules, it still can cause a signficant backup. Often we see it taking over a minute to process a message through MDSPAMD (even though the MDSPAMD logs show that the messages are actually moving through within a few seconds). Logs show they are just going through one at a time though. So there are just too many messages and they are backing up.